Along with an MI-26 heavy-lift helicopter, an An-72 cargo plane carrying half a dozen specialists from the Russian emergencies ministry has now landed on Spitzbergen, Russia's Itar-Tass news agency reported.

The temperature at the base was about -30C (-22F) with good visibility, forecasters said.

But the task ahead would be challenging even in optimum weather. Trying to locate the base in the almost featureless landscape could present problems. Rescuers will have only a few hours of gloomy half-light.

And if the ice is deemed too weak for the landing, the scientists will have to climb up ladders to get on.

If all goes to plan, the return flight should take just over six hours.

None of the scientists were hurt when parts of the installation sank on Wednesday, but they have lost most of their equipment and supplies.

The researchers have taken shelter in the two remaining buildings.

'Good mood'

The station's chief Vladimir Koshelyev told Russian NTV television spirits were high despite the danger.

The return flight from Spitzbergen should take about six hours

"We have sleeping spaces, we have heat going, we can live here, we have warm food ready.

"All the researchers are in a good mood, everything is all right," he said.

Russian officials said on Thursday the researchers had enough supplies to last about five days.

A nuclear-powered ice-breaker, the Arktika, is also heading to the station, and hopes to reach it within five days in case the air rescue fails, Russia's Itar-Tass news agency said.

Wall of ice

The break-up on Wednesday of the North Pole-32 base destroyed 90% of its structures in half an hour.

"All of a sudden a huge wall of ice appeared that kept growing and growing," Mr
Koshelyev told Russian media via telephone.

"First they were three metres (10 feet) high, then five, then seven and finally over 10.

"Such a thing, I would say, has never before been seen on a polar station."

North Pole-32 was set up 10 months ago on floating Arctic ice to study climate change and assess mineral reserves.

The drifting station has covered a distance of 2,750km (1,700 miles) since it was set up last April.